My jetta has always had a hard start, I've approached a few mechanics and they give different answers. My car has a rough idle and slight back fire whilst idling. Drives ok but I feel it could be better. I took it for a compression test and was told the intake manifold was leaking, when sorting that out I discovered that the exhaust manifold had a crack on it. I have replaced that and gaskets hoping it would solve the problem. Replaced motor mounts, carb kit, shocks and rack and tie rod ends. I still experience a lot of vibration and the idling is rough. It drops and escalates. Please help. What am I missing?